As a WLAN (Wireless Local Area Network) system works on an unlicensed spectrum, other users on a channel may share the spectrum. If multiple users send data simultaneously, mutual interference may occur and collision is caused. Currently, the WLAN system uses a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) mode to avoid collision. However, the CDMA/CA working mode is relatively inefficient. Especially, in a circumstance in which APs (Access Point) and STAs (Station) are deployed densely, a probability of collision from channel contention increases significantly, and MAC (Media Access Control) efficiency decreases significantly.
In an LTE (Long Term Evolution) system, to ensure high MAC efficiency, a scheduling-based transmission mode is used. In the scheduling-based transmission mode, user equipment does not need to acquire a transmit opportunity by means of channel contention, and a base station allocates time-frequency resources and performs scheduling for each user equipment to perform uplink and downlink transmission. Currently, the scheduling mode is used only on a licensed spectrum. Because there is no collision from channel contention, this mechanism can implement MAC efficiency of about 80%.
If the scheduling mode is introduced into a communications system that uses an unlicensed spectrum, some problems may be caused. For example, if the scheduling mode is introduced into a WLAN system, when two APs with an overlapping coverage area use the scheduling mode, work on a same channel, and cannot perform coordinated transmission, mutual interference may occur between the two APs and neither can work normally.